The present invention relates to a method and an arrangement for the formation of particles. More particularly, the present invention relates to a method for the formation of particles from a liquid, comprising the steps of forming droplets by means of an apparatus, which has a slinger means, which rotates in a direction of rotation and which has peripheral droplet-forming cusps, from which the liquid is slung in the form of droplets along diverging paths in a chamber which surrounds the apparatus and in which the droplets form solid particles, as well as an arrangement therefor.
The above-mentioned method and arrangement are normally used for the preparation of a powder from a liquid, said powder containing substantially spherical particles of usually equal size.
In the preparation of powder particles from certain types of liquid or melt, there are particles in the prepared powder, which have considerably smaller size than the other particles. The small particles are often referred to as dust particles and cause trouble when the powder is used, as the dust particles produce a great deal of dust, thus causing problems in the work environment.
Until now the reason for the development of the dust particles has been unknown, and therefore it has not been possible to solve the dust problem in a suitable manner at the stage of preparing the powder particles.
An object of the present invention is to provide a method and an arrangement for the formation of particles from a liquid, said method and said arrangement eliminating or, at any rate, alleviating said dust problems.
The present invention is based on the discovery that the dust particles, which sometimes form in the preparation of particles with the use of the above-mentioned method or arrangement, originate from threads between two successive droplets from one and the same droplet-forming cusp. Such a thread is stretched out when the two droplets are slung out. This is due to the fact that the droplets are slung in tangential direction from the same cusp but in different angular positions in the plane of rotation. Since the angular positions differ, the tangents in the different positions will describe two diverging paths, seen perpendicularly to the plane of rotation. Thus, the distance will increase between the droplets when they are slung out, whereby the thread formed between them is stretched out and finally separated from the two droplets. The thread will then contract and form a small droplet, a so-called satellite droplet. When the satellite droplet moves in the chamber, it dries and forms a dust particle.
Normally, in the preparation of particles use is made of a plurality of rotating droplet-forming disks which are piled in the vertical direction, whereby a considerable production volume can be achieved. By the discovery on which the invention is based, it has also been shown that the satellite droplets formed at the lower droplet-forming disks are the satellite droplets which are later found in the form of dust particles among the finished powder particles. The reason for this is that the satellite droplets formed at the upper droplet-forming disks are very likely to collide with the larger droplets and combine with the same. At the lower droplet-forming disks, the satellite droplets will normally have time to leave the xe2x80x9cswarmxe2x80x9d of droplets before such a collision takes places.
To achieve the object, the invention provides a method according to claim 1 and preferred embodiments are stated in dependent claims 2-7. In addition, according to claim 8 an arrangement is stated, and preferred embodiments appear from dependent claims 9-11.
More specifically, the invention provides a method for the formation of particles from a liquid, comprising the steps of forming droplets by means of an apparatus, which has a slinger means, which rotates in a direction of rotation and which has peripheral droplet-forming cusps, from which the liquid is slung in the form of droplets along diverging paths in a chamber which surrounds the apparatus and in which the droplets form solid particles, said method being characterised by the step of generating in the chamber an air flow, which is opposite to the direction of rotation of the slinger means and which, for the separation of dust particles, acts on threads formed of the liquid at the slinger means, satellite droplets formed of the threads, and the dust particles formed of the satellite droplets.
The term xe2x80x9cliquidxe2x80x9d shall be considered herein to comprise also various types of melts, the chamber, in the case of a liquid, preferably being heated for the droplets to dry and form solid particles, and, in the case of a melt, preferably being cooled for the droplets to solidify and form solid particles.
Since the droplets are slung out from the cusps of the slinger means in a tangential forward direction, seen in the direction of rotation of the slinger means, a thread formed between two droplets will be slung transversely of its own longitudinal direction. The air flow, which is directed opposite to this slinging direction, will act on a comparatively large surface of the thread and thus exert an air resistance on the same which considerably slows it down, and when the thread is subsequently separated from the droplets and forms a satellite droplet, the satellite droplet is further slowed down and can be entrained by the air flow. The dust particles formed of the satellite droplets are thus separated from the other particles. As a result, a method is provided, which eliminates or reduces at least to a considerable extent the above-mentioned dust problems in connection with the preparation of particles.
In particular, a thread can form between two successive droplets from each droplet-forming cusp, a satellite droplet can form when the successive droplets move along a diverging path each and thus stretch out the thread which is formed between them and which is thus separated from the droplets and contracts, and a dust particle can form of a satellite droplet as it moves in the chamber.
According to a preferred embodiment of the inventive method, the air flow rotates. The rotating air flow can, especially, be generated by separate partial flows, which are passed into the chamber and combined therein to form a common flow.
According to one embodiment of the inventive method, the dust particles separated from the air flow are passed to a collection means. Owing to this, it is possible to recirculate the collected dust particles to the apparatus for reuse.
According to a further embodiment of the method according to the invention, the slinger means of the apparatus comprises a plurality of axial levels, on which the formation of droplets takes place, the air flow primarily acting on the lower levels of the slinger means. As a result, the action of the air flow is concentrated to the levels from which the major part of the dust particles originate. As mentioned above, this is due to the fact that the satellite droplets on the higher axial levels are very likely to collide with other droplets and thereby combine with the latter. On the lower levels, the satellite droplets are more likely to have the time to separate from the xe2x80x9cswarmxe2x80x9d of droplets before such a collision takes place.
Moreover, the invention provides an arrangement for the formation of particles from a liquid, comprising an apparatus, which has a slinger means, which rotates in a direction of rotation and which has peripheral droplet-forming cusps, from which the liquid is slung in the form of droplets along diverging paths in a chamber which surrounds the apparatus and in which the droplets form solid particles, said arrangement being characterised by an air means, which is arranged to generate in the chamber an air flow, which is opposite to the direction of rotation of the slinger means and which, for the separation of dust particles, acts on threads formed of the liquid, satellite droplets formed of the threads, and the dust particles formed of the satellite droplets.
This arrangement causes an action on the dust particles and their previous stages, which action results in the separation of the dust particles. Thus, the arrangement allows the formation of particles, which eliminates or alleviates the dust problem described above.
The air means is specially arranged to generate an air flow, which acts on (i) threads, which are formed between two successive droplets from each droplet-forming cusp, (ii) satellite droplets, which are formed when the droplets move along a diverging path each and thus stretch out the thread formed between them, said thread thus being separated from the droplets and contracting, and (iii) dust particles, which are formed of the satellite droplets when they move in the chamber.
Preferred embodiments of the arrangement are stated in the dependent claims.